1. Field of the Invention
This invention relates to a single-phase brushless motor which is well adapted for a spindle motor for a floppy disk drive, a motor for audio and video appliances, and a DC brushless axial-flow fan motor.
2. Description of the Prior Art
A brushless motor is used in a variety of appliances, owing to its advantages of low noise and long life, because it has neither brush nor commutator, in addition to its characteristics as a DC motor.
In a brushless motor, an electronic circuit is used to switch energization of armature coils of the motor and includes a driving circuit which necessarily includes a number of position detecting elements (normally, magnetic sensors such as Hall effect elements and Hall ICs are used) corresponding to the number of applicable phases of the motor. Accordingly, it is a drawback that as the number of phases increases, the production cost increases accordingly, due to such position detecting elements.
Therefore, inexpensive appliances such as axial-flow fans commonly employ a single-phase brushless motor which includes a driving circuit designed for a single-phase and hence can be produced at a low cost.
Such a single-phase brushless motor has a drawback that it cannot start itself if it stops at a dead point.
Therefore, a conventional single-phase brushless motor is normally provided with special means such as a cogging torque (reluctance torque) generating member such as an iron bar located in an air gap or a special stator armature providing a slanted or sloped air gap for generating a cogging torque to assure self-starting of a magnet rotor of the motor.
Such conventional single-phase brushless motors utilizing a cogging torque as described above have a drawback that smooth rotation cannot be attained due to generation of a high cogging torque. Besides, the brushles motor employing an iron bar has a relatively large number of parts while the brushless motor employing a special stator armature cannot provide a high turning torque and is low in efficiency.
Single phase brushless motors are also known wherein a non-magnetized zone (or a substantially non-magnetized zone) is formed on a driving field magnet. Brushless motors of this type have a drawback that it is difficult and troublesome to form such a non-magnetized zone, which is not suitable for mass production of brushless motors of the type.